This invention relates generally to operational amplifiers and in particular to improvements in operational amplifier performance, and is more particularly directed toward a filter interposed between the gm stage and the integrator in a conventional two-stage operational amplifier architecture to improve the amplifier""s signal slew rate, gain, and harmonic distortion.
The conventional operational amplifier (op amp), illustrated in FIG. 1 in block diagram form, and generally depicted by the numeral 100, comprises two gain stages. The first functions as a differential transconductance (gm) stage 101 and the second as an integrator 103, separated by a differential to single-ended converter 102. The conventional op amp 100 is illustrated in more detail in FIG. 2.
As shown in FIG. 2, the gm stage 101 comprises a differential pair 201, 202 with a single current source xe2x80x9ctailxe2x80x9d 203 (both typically, and as an example, p-type insulated-gate field effect transistors), and two current source loads 204, 205 (typically, and as an example, provided by n-type transistors). By selecting an output 206 from only one of the differential input stages, differential to single-ended conversion is accomplished or, conventionally, current sources 204 and 205 are implemented as a mirror with single-ended output 206 derived from the high impedance side of the mirror.
This single-ended output 206 is then applied to the integrator stage 103. In the implementation shown, the integrator 103 includes a p-type output transistor 207 with a current source tail 210, and Miller capacitor 208. A nulling resistor 209 has been added for the sake of stability.
In conventionally designed op amps, the compensation capacitor 208 must be fairly large to forestall the possibility of oscillation. A large capacitor utilizes integrated circuit (IC) area that might otherwise be dedicated to other IC components. In addition, a large compensation capacitor negatively impacts amplifier gain at high frequencies and signal slew rate. Consequently, a need arises for an op amp design that provides improved high frequency gain and signal slew rate while reducing overall circuit area.
These needs and others are satisfied by the operational amplifier design of the present invention. The performance of an op amp is improved by placing a current mode filter between the gm stage and the integrator, which has a current gain of much less than one and is substantially without phase shift at the op amp""s resonant frequency, permitting stabilization with a relatively small compensation capacitor. This improves the signal slew rate and harmonic distortion.
In accordance with one aspect of the present invention, an improved operational amplifier includes a transconductance amplifier input stage and an integrator output stage. The improvement comprises a current-mode low pass filter interposed between the transconductance amplifier and the integrator.
In one form of the invention, the current mode low pass filter comprises a series resistor coupled between the transconductance amplifier output and the integrator input, a first capacitor connected in parallel with the resistor, and a shunt capacitor coupled between the transconductance amplifier output and ground.
In a preferred form, the series resistor has a resistance value of approximately 2.5 kilohms, the first capacitor connected in parallel with the resistor has a value of approximately 4 picofarads, and the shunt capacitor has a value of approximately 10 picofarads.
In accordance with another form of the invention, the improved operational amplifier further comprises a series damping resistor connected in series with the shunt capacitor. Preferably, the series damping resistor has a value of approximately 100 ohms.
Further objects, features, and advantages of the present invention will become apparent from the following description and drawings.